(1) Field of the Invention
The present invention relates to heat-developable color thermographic and photothermographic image-recording materials and more particularly to ones capable of providing images having good image discrimination as well as enhanced image density.
(2) Description of the Related Art
It is well known that various cleavage reactions are assisted by silver ions including reactions involving cleavage of a compound into one or more fragments.
U.S. Pat. No. 3,719,489 discloses silver ion assisted cleavage reactions useful in photographic systems. As disclosed therein, photographically inert compounds are capable of undergoing cleavage in the presence of silver ions made available imagewise during processing of a silver halide emulsion to liberate a reagent, such as, a photographically active reagent or a dye in an imagewise distribution corresponding to that of said silver ions. In one embodiment disclosed therein, color images are produced by using as the photographically inert compounds, color providing compounds which are substantially non-diffusible in the photographic processing composition but capable of undergoing cleavage in the presence of the imagewise distribution of silver ions and/or soluble silver complex made available in the undeveloped and partially developed areas of a silver halide emulsion as a function of development to liberate a more mobile and diffusible color-providing moiety in an imagewise distribution corresponding to the imagewise distribution of said ions and/or said complex. The subsequent formation of a color image is the result of the differential in diffusibility between the parent compound and liberated color-providing moiety whereby the imagewise distribution of the more diffusible color-providing moiety released in the undeveloped and partially developed areas is free to transfer.
Color-providing compounds useful in the above process form the subject matter of U.S. Pat. No. 4,098,783, a continuation in part of said U.S. Pat. No. 3,719,489. The color-providing compounds disclosed therein may comprise one or more dye radicals and one or more 1,3-sulfur-nitrogen moieties. For example, they may comprise one complete dye or dye intermediate and one cyclic 1,3-sulfur-nitrogen moiety. Alternatively, the color-providing compounds may comprise two or more cyclic moieties for each dye radical or dye intermediate and vice versa.
Thermally developable black and white as well as color photosensitive materials, whose development is effected by heating, are well known. Among the systems designed to give color images are those wherein a diffusible dye is released as a result of the heat development of an organic silver salt and transferred to the image-receiving layer whereby a color image is obtained.
Japanese Kokai 59-180548 having a Laid-Open date of Oct. 13, 1984 discloses a heat-developable silver halide photosensitive imaging system wherein the dye-providing material contains a heterocyclic ring containing a nitrogen atom and a sulfur or selenium atom which heterocyclic ring is subject to cleavage in the presence of silver ions to release a diffusible dye. An example of a suitable dye-providing material is a thiazolidine dye such as disclosed in the aforementioned U.S. Pat. No. 4,098,783. The process involves imagewise exposing the photosensitive system to light and subsequently or simultaneously heating the photosensitive system, in the presence of a base or base precursor, under a substantially water-free condition whereby an oxidation-reduction reaction between the exposed photosensitive silver halide and a reducing agent occurs. In the exposed areas a negative silver image is formed. In the unexposed areas, the silver ion, present in inverse proportion to the silver image, causes the heterocyclic ring of the dye-providing material to be cleaved releasing a diffusible dye. The diffusible dye is then transferred to an image-receiving layer whereby a positive dye image is formed.
Copending U.S. patent application Ser. No. 07/944,898 of J. R. Freedman et al, filed Dec. 22, 1992, describes a thermographic dye-transfer image-recording material wherein a silver salt complex is utilized as the source of silver ions made available upon imagewise heating to cleave a dye-providing material.
A heat-developable photosensitive system useful in terms of thermal development of the silver halide latent image is one which comprises a support carrying a photosensitive silver halide, a silver salt oxidizer, a thermal solvent, a reducing agent for the silver salt, a binder, preferably gelatin, and a dye-providing material capable of releasing dye upon silver ion assisted cleavage. However, in this type of system there has been difficulty in obtaining both adequate silver development and sufficient dye release.
It has now been found that, by including an auxiliary ligand capable of complexing with the silver ions of the silver salt oxidizing material in the imaging materials of the present invention, accelerated silver development, enhanced image density and improved image discrimination are obtained.